1. Field of the Invention
This invention relates to a head positioning control apparatus and a method therefor and more particularly, is suitably applied to a disc apparatus for recording on and reproducing from a disc recording medium, such as a magnetic disc, a magneto-optical disc, and an optical disc.
2. Description of the Related Art
A magnetic disc apparatus out of this type of disc apparatus is adapted to control the positioning of a magnetic head by sequentially switching three kinds of operation modes: a seek mode for rapidly moving the magnetic head to the vicinity of target position, a settling mode for settling the magnetic head at the target position, and a tracking mode for forcing the magnetic head to track the target position, in accordance with a positioning state of the magnetic head.
Particularly in a magnetic disc apparatus having a fixed medium, such as a Winchester disc, servo information is often written (hereinafter, this operation is referred to as the xe2x80x9cservo writexe2x80x9d ) after the apparatus is assembled. A disturbance produced in synchronism with a rotation of a disc at that time (hereinafter, this is referred to as the disc rotation synchronized disturbance) is not so large, so that it can be suppressed by ensuring a sufficient control bandwidth through a closed loop system using a proportional, integration and differential (PID) compensator, an H ∞ controller or the like upon tracking.
However, in a magnetic disc apparatus of a medium exchangeable type, such as a disc pack, a first-order component of disc rotation synchronized disturbance (eccentricity) generally varies whenever a medium is replace with another. Also, second-order and more disturbance components may often become large as compared with a magnetic disc apparatus of the fixed medium type, depending upon the rotation accuracy of a spindle motor or the like during the servo write.
More specifically, an ith component (i is a natural number) of disc rotation synchronized disturbance may occur when the disc suffers from eccentricity (i=1); when a track on the disc is deformed into an oval or indefinite shape (i=2); when a stamper, from which the disc is manufactured, has been deformed (ixe2x89xa73); and so on.
Further, from the fact that requirements to the head positioning accuracy has become more strict due to narrower track pitches, it is more and more difficult to ensure a sufficient suppression ratio for the disc rotation synchronized disturbance. For this reason, the introduction of a filter for suppressing the disc rotation synchronized disturbance has been proposed, wherein a sinusoidal wave generating model is inserted in a closed loop, in an application of an internal model principle, to increase the gain at its disturbance frequency to ensure the suppression ratio. As one of such filters for suppressing disc rotation synchronized disturbance, an adaptive feedforward canceller (AFC: Adaptive Feedforward Cancellation) has been proposed.
Here, a control system 1 using the AFC is illustrated in FIG. 1. This control system 1 is operative when a synchronized disturbance d(t) at a predetermined frequency is inputted to a controlled object P(s) to suppress the disturbance frequency using a digital AFC filter 2. First, when a periodic synchronized disturbance d(t) is inputted to the controlled object P(s) through an adder 3, the controlled object P(s) is provided with a component of the periodic synchronized disturbance d(t), and sends an output y(t) in accordance with the component to the outside and to the AFC filter 2.
Assuming that the frequency of this periodic synchronized disturbance d(t) is represented by xcfx89i/2xcfx80, the periodic synchronized disturbance d(t) is expressed by the following equation:
d(t)=Aicos(xcfx89it)+Bisin (xcfx89it)xe2x80x83xe2x80x83(1)
Subsequently, in the AFC filter 2, the output y(t) of the controlled object P(s) is provided to corresponding multipliers 3, 4, where the output y(t) is multiplied by cos (xcfx89it+"PHgr"i) and sin (xcfx89it+"PHgr"i), respectively. Then, the multiplication results are supplied to integrators 6, 7, respectively. The integrators 6, 7 integrate the multiplication results of the multipliers 4, 5, respectively, to produce AFC coefficients ai and bi, respectively. "PHgr"i represents the phase value of the frequency xcfx89i/2xcfx80 in the transfer function from an AFC addition point (u(t)) of the controlled object P(s) to an AFC draw-in point (y(t)).
The AFC coefficients ai and bi thus produced are multiplied by cos (xcfx89it) and sin (xcfx89it), respectively, in multipliers 8, 9 corresponding thereto, and then the respective multiplication results are added in an adder 10, with the addition result serving as an input u(t) to the controlled object P(s). This input u(t) is expressed by the following equation:
u(t)=aicos(xcfx89it)+bisin (xcfx89it)xe2x80x83xe2x80x83(2)
The adder 3 adds this input u(t) to the periodic synchronized disturbance d(t) to suppress a predetermined frequency component within the periodic synchronized disturbance d(t). In this way, a feedforward control using the AFC filter 2 as mentioned is repeated so that the AFC coefficients a and b are both converged to the AFC coefficients A and B represented in the periodic synchronized disturbance d(t), and consequently, the periodic synchronized disturbance d(t) is canceled by the input u(t) in the adder 3.
Actually, since the calculation processing performed by the AFC filter 2 (hereinafter, this is referred to as the xe2x80x9cAFC calculation processingxe2x80x9d ) is generally performed in a digital signal processor (DSP), the AFC coefficients a and b are updated in accordance with update rules expressed by the following equations, respectively:
ai(kT)=ai((kxe2x88x921)T)+giy(kT)cos(xcfx89ikt+"PHgr"i)xe2x80x83xe2x80x83(3)
bi(kt)=bi((kxe2x88x921)T)+giy(kT)sin(xcfx89ikt+"PHgr"i)xe2x80x83xe2x80x83(4)
where k is an integer indicative of a sampling time, and T is a sampling interval. In this event, the system function (transfer function) of the AFC filter 2, C(t) (=u(t)/y(t)), is expressed by the following equation:                                           C            i                    ⁡                      (            t            )                          =                              t            ⁡                          (                                                                    cos                    ⁡                                          (                                              Φ                        i                                            )                                                        ⁢                  t                                -                                  cos                  ⁡                                      (                                                                                            ω                          i                                                ⁢                        T                                            +                                              Φ                        i                                                              )                                                              )                                                          t              2                        -                          2              ⁢                              xe2x80x83                            ⁢                              cos                ⁡                                  (                                                            ω                      i                                        ⁢                    T                                    )                                            ⁢              t                        +            1                                              (        5        )            
Next, FIG. 2 illustrates a conventional magnetic disc apparatus 10. The magnetic disc apparatus 10 rotates a plurality of magnetic discs 11A and 11B at a high speed in accordance with the rotation of a spindle motor 12 for driving them, and simultaneously moves magnetic heads 14A to 14D mounted at respective tips of movable arms 13 in accordance with the driving of a voice coil motor (VCM) 15 to align them corresponding to one face 11AX, 11BX and the other face 11AY, 11BY of each magnetic disc 11A, 11B, so that data is recorded or reproduced by each of the magnetic heads 14A to 14D which follows respective tracks formed concentrically or spirally on the one face 11AX, 11BX and the other face 11AY, 11BY of each of the magnetic discs 11A, 11B.
Servo schemes for use in this magnetic disc apparatus 10 include a so-called embedded servo scheme, a servo face servo scheme, and so on. In the embedded servo scheme, a plurality of servo regions are formed such that they radially extend from the center of a disc to equi-angularly divide data regions, and servo information is embedded between the data regions. The servo face servo scheme, which is intended for a large capacity magnetic disc apparatus having a plurality of discs, specifies one face of one magnetic disc among them as a face dedicated to servo information, such that servo information is embedded entirely over the specified face.
With a servo scheme as mentioned, respective servo regions formed on the faces 11AX, 11BX and the other faces 11AY, 11BY of the magnetic discs 11A, 11B are formed with servo information serving as a time base, from which positional information can be provided for the magnetic heads 14A to 14D.
A reproduced signal S1 derived by reproducing servo information in the respective servo regions on the faces 11A, 11BX and the other faces 11AY, 11BY of the magnetic discs 11A, 11B by the magnetic heads 14A to 14D are amplified by a preamplifier 15, and converted into a digital form by an A/D converter 16 to generate a reference signal S2 which is sent to a position error signal generator 17.
The position error signal generator 17 generates a position error signal (PES) S3 representing how far the respective magnetic heads 14A to 14D deviate from their target tracks based on the reference signal S2, and sends the position error signal S3 to an adder 19 and a switch 20 in an AFC correction control system 18.
The AFC correction control system 18 supplies the error signal S3 to four AFC filters 21A to 21D, respectively, through the switch 20, executes the aforementioned AFC calculation processing to suppress the first-order to fourth-order components of a disc rotation synchronized disturbance, and then adds the outputs of the AFC filters 21A to 21D by an adder 22. The addition result is sent to the adder 19 through a switch 23 as an AFC output signal S4.
The switches 20, 23 are connected to an ON state only in a tracking mode, and are left in an OFF state in a seek mode or in a settling mode, other than the tracking mode, under the control of a mode switch signal S5 which is supplied from a control mode switching unit 24.
Assuming herein that the rotational frequency of the magnetic discs 11A, 11B is xcfx89/2xcfx80, the system functions C1(z) to C4(z) of the AFC filters 21A to 21D for canceling the first-order to fourth-order components of the disc rotation synchronized disturbance are expressed by the following equations, respectively:                                           C            1                    ⁡                      (            z            )                          =                              g            1                    ⁢                      xe2x80x83                    ⁢                                    z              ⁡                              (                                                                            cos                      ⁡                                              (                                                  Φ                          1                                                )                                                              ⁢                    z                                    -                                      cos                    ⁡                                          (                                                                        ω                          ⁢                                                      xe2x80x83                                                    ⁢                          T                                                +                                                  Φ                          1                                                                    )                                                                      )                                                                    z                2                            -                              2                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                          ω                      ⁢                                              xe2x80x83                                            ⁢                      T                                        )                                                  ⁢                z                            +              1                                                          (        6        )                                                      C            2                    ⁡                      (            z            )                          =                              g            2                    ⁢                      xe2x80x83                    ⁢                                    z              ⁡                              (                                                                            cos                      ⁡                                              (                                                  Φ                          2                                                )                                                              ⁢                    z                                    -                                      cos                    ⁡                                          (                                                                        2                          ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          T                                                +                                                  Φ                          2                                                                    )                                                                      )                                                                    z                2                            -                              2                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                          2                      ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      T                                        )                                                  ⁢                z                            +              1                                                          (        7        )                                                      C            3                    ⁡                      (            z            )                          =                              g            3                    ⁢                      xe2x80x83                    ⁢                                    z              ⁡                              (                                                                            cos                      ⁡                                              (                                                  Φ                          3                                                )                                                              ⁢                    z                                    -                                      cos                    ⁡                                          (                                                                        3                          ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          T                                                +                                                  Φ                          3                                                                    )                                                                      )                                                                    z                2                            -                              2                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                          3                      ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      T                                        )                                                  ⁢                z                            +              1                                                          (        8        )                                                      C            4                    ⁡                      (            z            )                          =                              g            4                    ⁢                      xe2x80x83                    ⁢                                    z              ⁡                              (                                                                            cos                      ⁡                                              (                                                  Φ                          4                                                )                                                              ⁢                    z                                    -                                      cos                    ⁡                                          (                                                                        4                          ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          T                                                +                                                  Φ                          4                                                                    )                                                                      )                                                                    z                2                            -                              2                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                          4                      ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      T                                        )                                                  ⁢                z                            +              1                                                          (        9        )            
The adder 19, on the other hand, adds the phase error signal S3 supplied from the phase error signal generator 17 and the AFC output signal S4 derived from the addition results of the four AFC filters 21A to 21D to generate an AVD correction signal S6.
Subsequently, a pair of switches 29, 30 each having three input and output terminals are disposed before and after a tracking controller 26, a settling controller 27 and a seek controller 28. The switches 29, 30 are switched in association with each other based on a mode switching signal S5 supplied from the control mode switching unit 24 such that an output terminal of one switch is connected to an input terminal of the other switch at the same position.
Thus, when the magnetic heads 14A to 14D are positioned at their respective target tracks, the AFC correction control system 18 switches the switches 20, 23 to an ON state only in the tracking mode and switches the switches 20, 23 to an OFF state in the subsequent seek mode and settling mode as the magnetic disc apparatus is switched sequentially to the seek mode, the settling mode and the tracking mode.
In this way, the AFC output signal S4 is sent to the adder 19 only in the tracking mode so that the adder 19 adds the AFC output signal S4 and the phase error signal S3 to generate the AFC correction signal S6 which is sent to the tracking controller 26.
The tracking controller 26 calculates head position information on the magnetic heads 14A to 14D based on the AFC correction signal S6 to generate a head driving signal S7 which is converted into an analog form by a D/A converter 31 and then sent to a voice coil motor driver 32. As a result, the voice coil motor driver 32 can drive a voice coil motor 33 based on the head driving signal S7 to make the magnetic heads 14A to 14D follow target tracks formed on the corresponding faces 11AX, 11BX, 11AY, 11BY of the magnetic discs 11A, 11B, respectively.
In the seek mode or the settling mode, on the other hand, the error signal S3 generated by the error signal generator 17 is supplied directly to the settling controller 27 or the seek controller 28. The settling controller 27 or the seek controller 28 respectively calculates head position information, and sends its calculation result to the voice coil motor driver 32 through a D/A converter 31 as a head driving signals S8 or S9. Consequently, the voice coil motor driver 32 can drive the voice coil motor driver 32 based on the head driving signal S8 or S9 to have the magnetic heads 14A to 14D seek or settle on target tracks formed on the corresponding faces 11AX, 11BX, 11AY, 11BY of the magnetic discs 11A, 11B, respectively.
In the magnetic disc apparatus 10 configured as described above, the AFC filters 21A to 21D execute the aforementioned AFC calculation processing based on the error signal S3 derived by reproducing the magnetic discs 11A, 11B and adds the AFC output signal S4 as the calculation result to the error signal S3 to sufficiently suppress a disturbance frequency which occurs in synchronism with the rotational frequency of the magnetic discs 11A, 11B derived from the error signal S3.
When the magnetic heads 14A to 14D are moved from tracks, on which the magnetic heads 14A to 14D are currently positioned under the tracking control, to desired target tracks, the tracking controller 26 instructs the seek controller 28 to perform mode switching to have the magnetic heads 14A to 14D seek from the currently positioned tracks to the target tracks. In this event, the AFC coefficients as the AFC calculation results by the AFC filters 21A to 21D are converged values derived when the tracking control was performed, and the seek controller 28 is supplied with the AFC correction signal S6 which has disc rotation synchronized disturbance corrected on the basis of the converged values.
Therefore, since the seek controller 28 has the magnetic heads 14A to 14D seek the target tracks before the disc rotation synchronized disturbance has not been completely corrected, it is likely that the magnetic heads 14A to 14D fail to settle to the target tracks even if the seek control is performed. This leads to a problem that the feedforward control using the converged AFC coefficients derived in the tracking mode becomes highly difficult in the seek mode.
The magnetic disc apparatus of the medium exchangeable type, on the other hand, is likely to suffer from very large eccentricity so that a compensation for disturbance of a particular frequency component is desired even in the seek mode. However, since the magnetic heads 14A to 14D move in the radial direction over the magnetic discs 11A, 11B, which are rotating at a high speed, during the seek mode, the AFC filters 21A to 21D cannot execute the aforementioned AFC calculation processing based on the error signal S3 to update the values of AFC coefficients, consequently resulting in a problem that considerable difficulties are encountered in compensating for a disturbance frequency which is in synchronism with the rotational frequency of the magnetic discs 11A, 11B.
Further, the disturbance frequency which was not able to be compensated for in the seek mode is likely to more adversely affect the positioning during the settling mode, causing a problem that the magnetic heads 14A to 14D cannot be positioned to desired target tracks on the magnetic discs 11A, 11B.
In view of the foregoing, an object of this invention is to provide a head positioning control apparatus and a method therefor which can significantly improve the head positioning accuracy with a simple configuration.
The foregoing object and other objects of the invention have been achieved by the provision of a head positioning control apparatus and a method therefor, in which position error signal generating means generates a position error signal indicative of the amount of positional deviation of a head with respect to a first target track on a disc recording medium, frequency correcting means generates frequency correction coefficients for correcting a disturbance frequency occurring in synchronism with a rotational frequency of the disc recording medium when a tracking control is performed on the basis of the position error signal, and head moving means moves the head from the first target track on the disc recording medium, on which the head is positioned as a result of the tracking control, to a neighboring position of a next second target track based on the position error signal and the frequency correction coefficients.
As a result, the head moving means moves the head from the first target track on the disc recording medium, on which the head is positioned as a result of a tracking control, to a neighboring position of the next second target track based on frequency correction coefficients derived from the frequency correcting means when the tracking control is performed on the basis of a position error signal, so that a disturbance frequency occurring in synchronism with the rotational frequency of the disc recording medium can be corrected even in an operation in which the head moving means causes the head to travel over a large amount of distance.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.